Phased array radar systems employ amplifiers and phase shifters to electronically steer radio frequency (RF) beams, in contrast to the more familiar rotating radar dish. By using phase shifts, phased array radar systems eliminate the time delay between signals that is typically associated with rotating systems as the dish rotates 360°. Thus, phased array radar systems result in improved performance.
One key parameter governing the RF performance of a phased array radar system, however, is the temperature or thermal gradient at all levels of the system. This is due to the fact that the RF performance changes with temperature. The gain and the phase shift of an amplifier each change with a change in temperature. The gain changes approximately 0.1 dB/° C. and the phase changes approximately 0.8°/° C. Thus, controlling thermal gradients in phased array radar systems is an important and challenging requirement to meet.
At the component level, heat sources such as integrated circuits and/or electrical components—for example the amplifiers in the transmit/receive (T/R) modules of the phased array radar system—must not only be cooled, but it is also important that the temperature gradient between any two such heat sources be minimized. One way to cool the amplifiers is to mount them on a cold plate which dissipates the heat from the amplifiers. Because the distance from each amplifier to the cooled edge of the cold plate varies, however, the amplifiers which are located the greatest distance from the cooled edge of the cold plate (e.g., the interior mounted amplifiers) operate at a higher temperature than the edge mounted amplifiers.
Various methods and systems have been devised to eliminate this level of thermal gradients. One system utilizes a cold plate assembly including inwardly directed tabs extending from opposing cooled edges of the cold plate. The tabs include orifices therethrough which are aligned with orifices in the skins of the cold plate, thus reducing the conductivity of the cooled edges of the cold plate. Consequently, the temperature gradient between edge mounted heat sources—amplifiers for example—and inwardly mounted heat sources is effectively reduced. See U.S. Pat. No. 6,903,931 incorporated herein in its entirety by this reference.
While this latter system and method, as well as others, have proven effective at cooling the components and reducing or eliminating the thermal gradient between the components of the T/R modules, additional thermal gradients arise at other levels of the phased array radar system.
For example, after the T/R modules including the foregoing cold plates and electrical components are attached to a T/RIMM (transmit/receive integrated multi-channel module), several hundred T/RIMMs may be combined to form the phased array. Each series of T/RIMMs are then in turn cooled by a plurality of cooling manifolds or heat sinks which are typically fluid cooled. Given the size of these arrays and the large number of T/RIMMs cooled along the way, the fluid flowing through the manifold undergoes an increase in temperature between the time of entry into and exit from the manifold. In other words, as the fluid flows, its temperature increases as it absorbs heat from the various T/RIMMs it serves to cool, resulting in hotter T/RIMMs located further downstream. Thus, an undesirable thermal gradient is generated at this level of the phased array radar system as well.
In an attempt to create an isothermal heat sink or manifold, most previous designs depend on vacuum brazed aluminum cooling manifolds with tailored heat transfer passages, cooling channels designed to take advantage of cross or counter flow, or by “shifting” the coolant away from some components or heat sources to effectively increase the temperature of devices when the coolant is the coldest. In one recent example, brazed aluminum manifolds have been used to convection/conduction cool electronic components by filling the cooling channels in the manifold with a fin stock that increases the heat transfer area at the expense of a greater fluid pressure drop.
These brazed manifolds/heat sinks provide distinct disadvantages, however.
One disadvantage is that the tailored heat passages provide a “calibration” that is only effective for a single heat dissipation. Given such a specific “calibration”, if the radar duty cycles are changed or if the system is put in a receive only mode, for example, the carefully calculated zero thermal gradient is thrown out of balance. Thus, thermal gradients may arise when a manifold with tailored passages is subjected to varying conditions.
Moreover, because of the complex cooling channel geometry; including tailored heat transfer passages, cooling channels designed to take advantage of techniques of cross or counter flow, or designs which “shift” coolant away from some components; brazed aluminum manifolds are expensive, typically have long lead times, and in some cases can be difficult to manufacture.
An additional disadvantage of such heat sinks or manifolds follows from that fact that vacuum brazing furnaces are limited in size. While furnaces are typically less than twelve feet long, larger arrays can be up to one hundred feet long. Thus, for larger arrays manufacturing becomes more difficult and expensive. Even with facilities large enough to braze some larger manifolds, special brazing expertise is required to build acceptable parts without significant repair and reworking.